Methods of preparing etch resists using an electrostatic image developer composition including a resin hardener



United States Patent This is a division of my copending applicationSerial No. 53,608, filed September 2, 1960.

This invention relates generally to improved methods of producing etchresists and more specifically to improved methods of preparing etchedprinting plates and printed circuits.

It is frequently desirable to mask selected areas of a surface forvarious purposes. For example, in the printing arts, a typographicprinting plate is often prepared by masking selected areas of thesurface of a sheet of metal, such as magnesium, zinc, or copper, with anacid or alkali resist and then etching the unmasked areas of the plateto a desired depth. In the electronic arts, etched circuits are commonlyprepared by masking selected areas of the surface of a metal-coateddielectric base with a resist and then etching away the unmaskedportions of the metal coating.

Photoetching techniques are commonly employed for producing typographicprinting plates and printed circuits. In conventional photoetchingprocesses, the object to be etched, usually a metal plate or metal cladsubstrate, is coated with a photoresist. The photoresist is then exposedto an ultraviolet light image, usually by a contact exposure, until theexposed photoresist is rendered insoluble or hardened in the exposedareas. The unexposed photoresist is washed away and the object etched tothe desired depth.

Conventional photoresists generally require relatively long exposures tothe ultraviolet light image because of the relative insensitivity ofavailable photoresists. Consequently, it becomes a practical necessityto first prepare a permanent, relatively dense, full-size transparencyof the subject matter to be etched. The transparency must then be heldfirmly against the photoresist during the relatively long, intenseexposure to ultraviolet light to obtain the necessary hardening of thephotoresist with a minimum loss in resolution. To accomplish this,photoetchers resort to special vacuum frames for holding thetransparency in place, and to intense ultraviolet light sources forexposure. Even so, exposure times of ten minutes or more are common.

It is an object of this invention to provide improved methods ofproducing etch resists on etchable surfaces.

It is another object to provide improved methods of electricallyproducing etch resists.

It is an additional object to provide improved methods of producingetched printing plates.

It is a further object to provide improved methods of producing printedcircuits.

It is still a further object to provide improved electrophotographicmethods of producing etched printing plates and printed circuits whereinthe need for employing relatively insensitive light-hardenable materialsis obviated.

These and other objects and advantages are accomplished by thisinvention which includes methods of processing etchable plates having onone surface thereof a coating comprising an insulating resinous layer. Alatent electrostatic image is produced on the coating and is developedwith a toner powder at least a portion of which comprises across-linking agent or catalyst for the resinous layer. Once a powderimage is developed on ice the resinous layer, it is heated tocatalytically cure the resin layer underlying the powder image toconvert it into an etch resist.

In a preferred method of this invention, an etchable plate is providedwhich has on one surface thereof a photoconductive insulating layerhaving a resinous component which has molecular chains capable of beingcross-linked. A latent electrostatic image is electrophotographicallyproduced on the photoconductive coating and is developed with acatalytic developer powder. As before, once the powder image is heated,the underlying resin is converted into an etch resist.

This invention also includes improved developer materials capable ofpromoting cross-linking in resins. Such materials comprisefinely-divided electroscopic particles which include a resinousthermoplastic material and a cross-linking agent or catalytic material.

Specific examples and additional advantages of the improved methods ofthis invention are included in the following detailed description.

In accordance with one method of this invention, an etchable plate, suchas, for example, magnesium, zinc, or copper has on its surface aresinous coating. A coating resin is selected which is capable ofretaining an electrostatic charge and which includes molecular chainscapable of being cross-linked at elevated temperatures in the presenceof a suitable catalyst to provide on the plate a cured etch resist. Theresin is also selected to be soluble in a selected solvent in theuncured state and insoluble therein in the cured state. Suitable resinsinclude the following: vinyl resins, silicones (or resinouspolysiloxanes), phenolformaldehyde resins, polystyrenes, alkyd resins,amino resins, high styrene-butadiene resins, and compatible mixturesthereof. Such resins may be dried or cured in air or more rapidly byheat with a catalyst such as a peroxide or a linoleate, naphthanate,octoate, resinate, stearate or tallate of aluminum, cadmium, copper,iron, lead, manganese or zinc. Coating resins and catalysts therefor aremore fully described in Organic Coating Technology, vol. 1, by Payne,Wiley and Sons Inc., New York, N.Y.

A latent electrostatic image may be produced on the resin coating, forexample, by superimposing thereon a stencil and thereafter exposing thelayer through the stencil to an ion producing source such as an array ofcorona generating wires. The electrostatic image so produced is thendeveloped with finely-divided electroscopic thermoplastic particles. Ifthe developer particles have a triboelectric polarity different from thecharge polarity of the electrostatic image they will be attracted to thecharges on the coating to produce thereon a direct or positive powderimage. If the polarities are the same, the developer particles will berepelled by the charges and will deposit in uncharged areas to produce areverse or negative powder image. Various other methods for producingand developing electrostatic images are described in Electrofax DirectElectrophotographic Printing on Paper, by C. J. Young and H. G. Graig,RCA Review, December 1954, vol. XV, No. 4.

Developer powders made in accordance with this invention compriseparticles of electroscopic thermoplastic material which include fromless than 2% to above by weight of resin curing catalytic material. Thecatalytic material may be imbedded in the thermoplastic material orparticles of catalytic material may be coated with the thermoplasticmaterial. Some catalytic powders in and of themselves have appropriatetriboelectric properties for developing electrostatic images. However,the choice is limited since those which have such properties may not bedesirable catalysts for the resin which is to be cured. By combining aresin with a catalyst, it is possible to produce developer powders whosetriboelectric properties are primarily determined by the resin componentand, hence, the catalyst can then be one which is more desirable from acuring standpoint.

Many natural and synthetic resins, waxes and other lowmelting materialscomprise suitable. thermoplastic components for the developer powders ofthis invention. For example, any of the following materials forcombinations thereof may be used:

If desired, various pigments or dyes may be included in the developerparticles of this invention.

A suitable brown developer powder may be prepared as follows:

Example I 90 grams finely-divided Piccolastic resin 4358 (an elasticthermoplastic resin composed of polymers of styrene, substituted styreneand its homologs) marketed by the Pennsylvania Industrial Chemical Corp.

grams aluminum octoate.

These materials are thoroughly mixed and melted in a stainless steelcontainer at 200 C. Mixing is accomplished in as short a time aspossible. The melt is then poured onto a brass tray to cool and harden..

The hardened mix is then broken up and ball milled for about hours, Themelted powder is screened through a 200 mesh screen and is then readyfor use as a developer powder. This powder takes on a positiveelectrostatic chargewhen mixed with carrier beads or iron powder asdescribed in the aforesaid Young and Greig publication. It thereforedevelops an electrostatic image composed of negative charges.

Once the electrostatic image has been developed with the developermaterial, heat is applied to fuse the developed image to the resincoating and to cure the resin underlying the image.

Heating, at a temperature of about 200 to about 250 degrees centigradefor about one minute, is generally sufli-cient to cure the coating resinand convert it into an etch resist.

A solvent for the coating is now applied to the surface of the imagecarrying plate in order to remove the nonresi-st (uncured) areas of thecoating, while those areas of the coating which have been cured are leftundisturbed. Suitable solvents for a silicone resin coating are methyl,ethyl, butyl or isoamyl alcohols, diacetone, toluene and xylol. Wherethe solvent merely softens the coating, a cotton swab, for example,dampened with the solvent is applied to the surface wiping away theareas that have been softened.

- Once curing of the coating pattern is completed and uncured areas ofthe coating removed, the plate is ready for etching with a standard etchsolution. In the preparation of a printing plate, etching is continueduntil the desired depth is attained. In the preparation of a printedcircuit, the plate to be etched comprises a metal layer bonded to adielectric base and etching is continued until all metal in uncuredareas on the dielectric base is removed. From the foregoing descriptionit is obvious that this invention is equally adapted for preparingplates other than printing plates and printed circuits. Ornamentalplaques and name plates are examples of such other plates.

Toner powders may be prepared as set forth in Example I but withdifferent proportions of catalyst and resin.

Toners with the following proportions will also provide. satisfactoryresults in the methods of this invention.

Example II Grams Aluminum octoate 2.5 I Resin (Piccolastic 4358) 97.5Example III Aluminum octoate 5 Resin (Piccolastic 4358) Example IVAluminum octoate 2O Resin (60 grams Piccolastic and 40 grams gramsPiccolastic 100 Difficulty has been experienced in producing toner pOW-ders by the process of Example I which include more than 10 or 20% byweight of catalyst, This difficulty, however, is easily overcome bypreparing solutions of resin and catalyst as follows:

Example V Grams Aluminum octoate Resin ('Piccolastic 4358) 20 Toluene 50The catalyst and the resin are dissolved in the toluene and poured ontoa metal or glass surface. Once all the toluene has evaporated, a brittlemass of thoroughly mixed resin and catalyst is obtained. This mass isbroken up, ground, and classified as to particle size as in Example I.Toner powders which include up to at least 80% catalyst may be preparedin this manner. In the preferred methods of this invention,electrophotographic processes such as are described in theaforementioned Young and Greig publication are employed. In suchprocesses, an etchable plate is provided which has on one surfacethereof a photoconductive insulating coating which includes a resincomponent. As described in the above publication, one such coatingcomprises a photoconductive zinc oxide dispersed in a resin binder.Other suitable photoc-onductors are described in U.S. Patent 2,862,815to M. L. Sugarman and A. J. Moncrieff- Yeates. The requirements withrespect to the resin component for such a photoconductive coating aresubstantially the same as those for the insulating coating resin earlierdescribed herein. Such a resin component should be one in which afinely-divided photoconductor is easily dispersed and such as to providea stable dispersion in the final coating. It is essential that the resinbe a good insulator and that it have molecular chains which can becross-linked in the presence of a selected catalyst.

In the electrophotographic processing of a plate having aphotoconductive coating, it is usually convenient to first provide onthe coating a uniformly distributed electrostatic charge. Such achargemay be produced by passing over the coating a corona generatingdevice compris ing an array of fine wires connected to a source of highpotential.

Once the coating is charged, it is then exposed to a light image.Exposure may be made by conventional contact printing or projectiontechniques. Exposure to: the light image produces on the coating alatent electrostatic image, the charged areas of which correspond to thedark areas of the original light image.

The latent electrostatic image may be developed into a powder image,asdescribed heretofore, or, in the alternative, development may beaccomplished by liquid de-- velopment techniques. Such techniques mayinclude ap-- plying to an electrostatic image a dispersion comprising; afinely-divided thermoplastic resin toner dispersed in an: insulatingliquid, the toner including a catalyst as described heretofore. Suitableinsulating liquids include various-v hydrocarbons as well astrichlorotrifluoroethane and a.

low viscosity dimethyl polysiloxane or mixtures thereof.

What is claimed is:

1. A method of processing an etchable plate having on one surfacethereof a photoconductive insulating layer comprising a finely-dividedphotoconductor dispersed in a binder of insulating resinous materialwhich is soluble in a selected solvent and which includes molecularchains capable of being cross-linked with a selected cross-linking agentwhen heated to render the resinous material insoluble in said solvent;said method comprising the steps of: producing a latent electrostaticimage on the surface of said resinous layer; developing saidelectrostatic image into a powder image with a finely-dividedthermoplastic material at least a portion of which comprises saidselected catalyst; and heating said powder image to cure the portions ofsaid resinous material underlying said powder image to convert saidportions into an etch resist.

2. The method of claim 1 including the additional step of etching saidplate to a desired depth.

3. The method of claim 1 wherein said plate comprises a metal layerbonded to an insulating base; said method including the additional stepof etching away all portions of said metal layer not covered by saidetch resist to leave a conductive pattern on said insulating base.

4. A method of producing an etch resist on an etchable plate having aphotoconductive insulating coating thereon comprising a finely-dividedphotoconductor dispersed in a binder of resinous material which issoluble in a selected solvent and which includes molecular chainscapable of being cross-linked by heat with a selected crosslinking agentto render the resinous material insoluble in said solvent; said methodcomprising the steps of: electrophotographically producing a latentelectrostatic image on said coating; developing said electrostatic imageinto a powder image with a finely-divided thermoplastic material atleast a portion of which comprises a cross-linking agent for saidresinous material, and heating said powder image to cure the portions ofsaid resinous material of said coating underlying said powder image toconvert said portions into an etch resist.

5. The method of claim 4 wherein said etch resist is produced on thesurface of a metal layer bonded to an insulating base and all portionsof said metal layer not covered by said resist are etched away toprovide a conductive pattern on said insulating base.

-6. The method of claim 4 wherein said etch resist is produced on thesurface of a raw printing plate and all areas on said plate not coveredby said resist are etched to a desired depth.

7. A method of producing an etch resist on an etchable plate having aphotoconductive insulating coating thereon comprising a finely-dividedphotoconductor dispersed in a binder of insulating resinous materialwhich is soluble in a selected solvent and which includes molecularchains capable of being cross-linked by heat in the presence of aselected cross-linking agent to render to resinous material insoluble insaid solvent; said method comprising the steps of:

electrophotographically producing a latent electrostatic image on saidcoating;

developing said electrostatic image into a powder image with afinely-divided thermoplastic material a substantial portion of whichcomprises said cross-linking agent;

heating said coating with said powder image thereon to cross-link saidresinous material in all portions of said coating which underly saidpowder and render such material insoluble in said selected solvent; and

applying said selected solvent to said coating to remove saidphotoconductor and said binder from all areas on said plate not coveredby said powder.

8. A method of producing an etch resist on an etchable plate having aphotoconductive insulating coating thereon comprising a finely-dividedphotoconductor dispersed in a binder of insulating resinous materialwhich is soluble in a selected solvent and which includes molecularchains capable of being cross-linked by heat in the presence of aselected cross-linking agent to render said resinous material insolublein said solvent; said method comprising the steps of:

electrophotographically producing a latent electrostatic image on saidcoating;

developing said electrostatic image into a powder image with developerparticles comprising thermoplastic resinous material and 2 to percent byweight of said cross-linking agent; heating said coating with saidpowder image thereon to cross-link said resinous material in allportions of said coating which underly said powder and render suchmaterial insoluble in said selected solvent; and

applying said selected solvent to said coating to remove saidphotoconductor and said binder from areas on said plate not covered bysaid powder;

etching said plate to a desired depth in areas thereof from which saidphotoconductor and binder have been removed.

9. A method of processing an etchable plate having a photoconductivecoating thereon comprising a finely-divided photoconductor dispersed ina resinous polysiloxane binder which is soluble in a selected solventand which includes molecular chains capable of being cross-linked in thepresence of aluminum octoate and heat to render said binder insoluble insaid solvent; said method comprising the steps of:

electrophotographically producing an electrostatic latent image on saidcoating;

developing said latent image into a powder image with developerparticles comprising thermoplastic resinous material and 2 to 80 percentby weight of aluminum octoate;

heating said coating with said powder image thereon to efiectcross-linking of said resinous material in all portions of said coatingswhich underly said powder and render such material insoluble in saidselected solvent; and

applying said solvent to said coating to remove said photoconductor andbinder from areas on said plate not covered by said powder.

References Cited by the Examiner UNITED STATES PATENTS 2,579,332 12/51Nelson 26037 2,857,272 10/58 Grieg 961 2,907,674 10/59 Metcalfe et al96-1 X 2,919,179 12/59- Van Wagner 96-1 X 2,939,787 6/60 Giamo 96--13,041,169 6/62 Wielicki 96-1 3,050,019 l0/62 Johnson et al. 96-13,082,181 3/63 Brown et al 260-37 OTHER REFERENCES Morgan: PaintManufacture, July 1951, vol. XXI, No. 7, pp. 239-248 and 259.

NORMAN G. TORCHIN, Primary Examiner.

4. A METHOD OF PRODUCING AN ETCH RESIST ON AN ETCHABLE PLATE HAVING APHOTOCONDUCTIVE INSULATING COATING THEREON COMPRISING A FINELY-DIVIDEDPHOTOCONDUCTOR DISPERSED IN A BINDER OF RESINOUS MATERIAL WHICH ISSOLUBLE IN A SELECTED SOLVENT AND WHICH INCLUDES MOLECULAR CHAINSCAPABLE OF BEING CROSS-LINKED BY HEAT WITH A SELECTED CROSSLINKING AGENTTO RENDER THE RESINOUS MATERIAL INSOLUBLE IN SAID SOLVENT; SAID METHODCOMPRISING THE STEPS OF: ELECTROPHOTOGRAPHICALLY PRODUCING A LATENTELECTROSTATIC IMAGE ON SAID COATING; DEVELOPING SAID ELECTROSTATIC IMAGEINTO A POWDER IMAGE WITH A FINELY-DIVIDED THERMOPLASTIC MATERIAL ATLEAST A PORTION OF WHICH COMPRISES A CROSS-LINKING AGENT FOR SAIDRESINOUS MATERIAL, AND HEATING SAID POWDER IMAGE TO CURE THE PORTIONS OFSAID RESINOUS MATERIAL OF SAID COATING UNDERLYING SAID POWDER IMAGE TOCONVERT SAID PORTIONS INTO AN ETCH RESIST.